Sensibilidad del modelo del proceso Caníbal, planta “Dulces Nombres”

The controlled environment experiments were conducted in covered containers as previously described (Gamliel and Stapleton, 1993) to test the effects of soil amendment with several preparations of gramineous plant residues and two temperature regimens. Survival and activity of the plant pathogens M. incognita, S. rolfsii, and P. ultimum was evaluated. The soil used was Hanford fine sandy loam (46% sand, 45% silt, 9% clay; pH 7.4) naturally infested with M. incognita (ca. 150 second-stage juveniles [J2] per liter of soil) and P. ultimum (ca. 29 propagules [oospores] per gram of soil). Laboratory-grown sclerotia of S. rolfsii were added to the soils in mesh bags (30 sclerotia per bag) prior to treatment. Soil for treatment was loaded into widemouthed, 2 l capacity glass jars sealed with metal lids. Four sub-samples of soil per test were treated by incubation in a single modified, Wisconsin-type waterbath with sub-lethal diurnal temperature maximum and minimum of 38o_{C and 27}o_{C, respectively, while others} were maintained in a similar waterbath set at ambient temperatures of 21-24o_C.

Gramineous Amendments

The gramineous amendments used in these tests were dried and Wiley-milled vegetative tissues of: wheat (Triticum aestivum L. cv. Yolo), barley (Hordeum vulgare L. cv. UC 337), oats (Avena sativa L. cv. Montezuma), and triticale (X Triticosecale Wittm.). All amendments were incorporated into soil at a concentration of 1.9% (w/w), an approximate quantity of residues tilled into soil at the end of a cropping cycle in commercial field production. Determination of Treatment Effects

Effects of treatments on M. incognita were estimated after 7 days using a bioassay procedure. Treated soil was aired in open plastic bags for 24 hr following the incubation period, then placed in two 10-cm-diameter pots per replication. A single plant of a susceptible tomato cultivar (Lycopersicon esculentum L. cv. Cherry Belle) was transplanted into each pot and the pots were maintained in a glasshouse at 30o_{C maximum and 21}o_{C minimum.}

After six weeks growth, roots were washed and an arbitrary gall rating was made (0-4 scale, where 0 = no galls evident and 4 = 75-100% of roots galled by visual examination). Some evidence of phytotoxicity arising from the soil amendments was noted in the bioassayed tomato plants. Sensitivity of S. rolfsii to treatments was determined after 7 days incubation, as previously described (Gamliel and Stapleton, 1993), by retrieving and surface- disinfesting the 30 sclerotia from the experimental containers, then placing them on PDA agar plates to determine germinability (Gamliel and Stapleton, 1993). After the 7 day treatment period, effects on P. ultimum were determined by sampling soil from containers, then air-drying and plating it on a selective medium as previously described (Gamliel and Stapleton, 1993). At least two sets of assays were performed for each amendment and soil heating combination tested.

Data Analysis

The in vitro experiments were used as replications for comparing amendments and temperature regimens. For each experiment, subsamples were averaged for each amendment by combining results from the warmer/cooler temperature treatments. All amendments were not included in each experiment, but each experiment had a nonamended control treatment. Consequently, the number of replicates differed among treatments. Data for pathogen survival and activity taken as percentages of the control treatments were transformed, using the arcsin of the square root or the base 10 logarithm to homogenize the variances. Analysis of variance was run for just the controls to compare experiments. Analysis of variance was also run to test the effects of amendments, temperature regimens, and their interactions. Contrasts of interest were calculated to test amendment versus control, and the interaction of amendment versus control and temperature. Fisher's Protected LSD test was used for mean separation of amendments for the cooler and warmer temperature regimens.

RESULTS

Analysis of variance for controls showed that experiments were not significantly different for galling of tomato roots by M. incognita or for germination of S. rolfsii sclerotia after 7 days. Experiments were significantly different for P. ultimum survival; therefore survival of P. ultimum after exposure to the various amendments for 7 days was calculated relative to the corresponding nonamended control at ambient temperature for the respective experiment. Post-treatment population assays of each of the test organisms confirmed that the warmer temperature regimen used during the experiments was sublethal. For each of the test pathogens, significant main effects for amendments, soil temperature, and the amendment x temperature interaction were found, with the exception that the interaction was not significant for S. rolfsii. The other consistently significant statistic across the range of pathogens was the contrast of amendment versus the control x temperature. Since the amendment x temperature treatment interactions and/or the amendment vs. control x temperature contrasts were significant, LSD tests were used for mean separation of amendments for the warmer and cooler treatments.

Addition of wheat and barley amendments alone gave significant reductions in root galling of tomato due to M. incognita, compared to the cooler temperature control, after 7 days treatment. Gall ratings were decreased by 99-100% when any of the amendments were incubated at the higher temperature regimen, with a significant treatment interaction between amendments and temperature. Addition of gramineous amendments at ambient temperature reduced germination of S. rolfsii sclerotia after 7 days incubation. Soil amendment with wheat caused the most deleterious effect on sclerotial germination. Incubation at the warmer temperature regimen completely inhibited germination.

Soil amendment with each of the gramineous residues except triticale significantly reduced numbers of P. ultimum after 7 days incubation at the cooler soil temperature. Wheat, barley, and oats were equally effective. When the amendments were combined with the sublethal heating regime, the reductions increased to 84-100%.

DISCUSSION

Biofumigation has the potential to significantly contribute to economic control of soilborne plant pests. However, the effects are often inconsistent or insufficient. In the high value, intensively-farmed horticultural crops common to California agriculture, it is unlikely that periodic rotations into bioactive, gramineous plants will provide sufficiently effective or predictable soil disinfestation. Addition of a soil heating component by way of solarization can assist in fulfilling the requirements of growers. Therefore, combination of gramineous soil amendments with solarization is a feasible option for development and implementation. However, since phytotoxicity has been observed following incorporation of gramineous crop residues, thorough testing will need to be done to provide sufficient guidelines for commercial use.

ACKNOWLEDGEMENTS

McKenry for kind provision of nematode analysis resources. Carol Adams provided statistical consultation.

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A LARGE-SCALE DEMONSTRATION OF SOLAR INACTIVATION OF INVASIVE WEED